1. Field of the Invention
The present invention relates to a zoom lens system for a digital still camera or a video camera, and in particular, relates to a miniaturized and high-performance zoom lens system having a zoom ratio of about 3.6.
2. Description of the Prior Art
As to a zoom lens system, a retro-focus lens system including a negative lens group and a positive lens group, in this order from the object, is easy to be constituted; however, the zoom ratio is limited to about 3. Unlike the above-explained retro-focus lens system, a four-lens-group zoom lens system including a positive lens group, a negative lens group, a negative lens group and a positive lens group, in this order from the object, has been known as a lens system which can increase the zoom ratio. On the other hand, in a four-lens-group arrangement, the number of lens elements thereof becomes larger, and the first lens group having a large diameter is used as a focusing lens group. As a result, the overall length of the lens system has to become longer, and such an arrangement is disadvantageous for a lens system with a function of automatic focusing.
Generally, with respect to size and optical performance (zoom ratio), a three-lens-group zoom lens system having a positive lens group, a negative lens group and a positive lens group, in this order from the object, is classified between a two-lens-group lens system and a four-lens-group lens system. In particular, a three-lens-group lens system which is disclosed in U.S. Pat. No. 5,912,771 filed by the applicant of the present invention has attained a zoom ratio of 3, while miniaturization and high-performance of the lens system have been attained. More concretely, in this three-lens-group zoom lens system, the first lens group is made immovable, and zooming is performed by the second and third lens groups.
It is an object of the present invention to provide a miniaturized and high-performance four-lens-group zoom lens system for a digital still camera or a video camera, the lens system of which can attain a zoom ratio of about 3.6, while the size of the lens system and the number of lens elements thereof are substantially the same as those of a three-lens-group zoom lens system.
In order to achieve the above-mentioned object, there is provided a zoom lens system including a positive powered (hereinafter, positive) first lens group, a negative powered (hereinafter, negative) second lens group, a positive third lens group, and a positive fourth lens group, in this order from the object. Upon zooming from the short focal length extremity to the long focal length extremity, at least the first, second and third lens groups are moved so that the distance between the first lens group and the second lens group increases, the distance between the second lens group and the third lens group decreases, and the distance between the third lens group and the fourth lens group increases. The lens system satisfies the following conditions:
0.10 less than fw/f1 less than 0.30xe2x80x83xe2x80x83(1)
0.25 less than fw/f4 less than 0.45xe2x80x83xe2x80x83(2)
0.60 less than X3/(ftxe2x88x92fw) less than 0.85xe2x80x83xe2x80x83(3)
wherein
fw designates the focal length of the entire zoom lens system at the short focal length extremity;
ft designates the focal length of the entire zoom lens system at the long focal length extremity;
fi designates the focal length of the ith lens group (i=1 to 4); and
X3 designates the traveling distance of the third lens group from the short focal length extremity to the long focal length extremity.
The first lens group can be constituted by a positive single lens element only. With this arrangement, the zoom lens system satisfies the following condition:
0.20 less than (d2txe2x88x92d2w)/(ftxe2x88x92fw) less than 0.50xe2x80x83xe2x80x83(4)
wherein
d2w designates the distance between the first lens group and the second lens group at the short focal length extremity; and
d2t designates the distance between the first lens group and the second lens group at the long focal length extremity.
The fourth lens group can be constituted by a positive single lens element only, and can function as a focusing lens group. With this arrangement, the fourth lens group satisfies the following condition:
0.6 less than ft/ft1xe2x80x943 less than 0.8xe2x80x83xe2x80x83(5)
wherein
ft1xe2x80x943 designates the focal lengths from the first lens group to the third lens group, at the long focal length extremity.
The fourth lens group can be movable upon zooming. On the other hand, if the fourth lens group is made immovable, the mechanical structure of a lens barrel can be simplified.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-397823 (filed on Dec. 27, 2000) which is expressly incorporated herein in its entirety.